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Why does coronary blood flow increase during diastole?

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Illustration of a human heart cross-section showing four chambers-left and right atria and ventricles-with labeled major blood vessels. An inset highlights cardiac muscle tissue, showing myocytes and intercalated disks. — human cardiovascular system The human heart is both a pump and a muscle. Its four chambers move blood efficiently, while cardiac muscle cells joined by intercalated disks synchronize each contraction.

Coronary blood flow increases during diastole primarily because of the mechanics of the coronary circulation.

The two coronary arteries branch from the aorta just beyond the semilunar valves. During systole, the contraction of the heart muscle compresses the intramyocardial vessels, which increases resistance to blood flow. However, during diastole, the heart muscle relaxes, and the pressure within the aorta—specifically the pressure above the semilunar valves—forces oxygen-rich blood into the coronary arteries and into the musculature of the heart.1